Rewrite progress component without <progress> element

- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.

scss/mixins/_progress.scss

@@ -1,16 +0,0 @@
-// Progress bars
-
-@mixin progress-variant($color) {
-  &[value]::-webkit-progress-value {
-    background-color: $color;
-  }
-
-  &[value]::-moz-progress-bar {
-    background-color: $color;
-  }
-
-  // IE10+, Microsoft Edge
-  &[value]::-ms-fill {
-    background-color: $color;
-  }
-}